The present invention relates to the computed tomography and medical imaging arts. It particularly relates to computed tomography (CT) imaging using an x-ray source that produces a conical beam (conebeam CT) and which traverses a helical orbit relative to an imaging subject, and will be described with particular reference thereto. However, the invention will also find application in conjunction with other types of CT imaging including multi-slice imaging, fan-beam CT, and the like, as well as in conjunction with other imaging techniques.
Computed tomography (CT) imaging has application in many areas involving non-invasive examination of internal features of a subject. For example, CT is applicable in security baggage examination and in clinical and diagnostic medical imaging. Regarding medical applications, CT has been employed for cardiac imaging, functional imaging of dynamically moving organs such as the lungs, blood perfusion imaging, and for other types of medical imaging. CT advantageously provides three-dimensional clinical imaging without injected radiopharmaeceuticals. However, CT sometimes suffers from limited acquisition speed. As an example, in a modern CT the x-ray source typically orbits the subject at about 120 rpm, corresponding to 0.25 seconds for data acquisition over 180xc2x0. Since a complete cardiac cycle period is about one second or less, the CT acquisition time can lead to motion blurring in cardiac imaging.
In CT imaging, an x-ray source transmits x-rays into an examination region where the x-rays are partially absorbed by a subject being imaged. Detectors arranged across the examination region from the x-ray source detect the x-rays after passing through the examination region. The detected x-ray intensity is characteristic of the absorption experienced as the x-rays pass through the examination region, and the output data is typically arranged in a projection data format. Using appropriate mathematical techniques, the projection data is reconstructed into an image representation characteristic of the subject or a portion thereof.
In early CT imaging, the x-ray source was narrowly collimated into a beam or thin wedge-shaped slice. The x-ray source was rotated about the examination region in a circular orbit, and the resulting projection data was reconstructed into a slice image. The subject was repetitively stepped through the examination region in a direction perpendicular to the slice plane to acquire a plurality of such image slices. Taken together, a stack of slices provides a three-dimensional characterization of the subject. Although this CT configuration simplified many mathematical aspects of the image reconstruction, it was slow. Furthermore, the collimation of the x-ray source greatly reduced the x-ray power, resulting in lowered signal-to-noise ratios.
Recently, CT imaging systems have been developed in which a conebeam x-ray source moves along a helical path obtained by simultaneously orbiting the x-ray source while advancing the subject. The conebeam x-ray source is coupled with a two-dimensional array of x-ray detectors which record conebeam transmission across the area of the conebeam. Helical conebeam CT continuously acquires divergent two dimensional projection data which greatly improves scanning speed, and the less aggressive collimation makes more efficient use of the x-ray source output.
However, the reconstructed helical conebeam CT image quality has in the past been degraded by image artifacts and other degradation modalities resulting from the poorly defined x-ray path geometry. Past reconstruction methods have also typically included image-degrading approximations which however were incorporated to allow the reconstruction speed to keep pace with the rapid data acquisition of the helical conebeam geometry.
For example, while a thin wedge-shaped x-ray beam (typically spanning four slices) can be treated as parallel x-ray paths, this approximation with wider conebeam CT data is complicated by the three-dimensional extent of the conical x-ray beam. With a helical source path, there is no single plane containing all the projection data intersecting a selected voxel or image plane in the examination region. Rather, the conebeam rays pass through each voxel at a myriad of different angles and directions, and occur when the x-ray source is at different angular and longitudinal positions. For exact reconstruction a computationally intense three-dimensional reconstruction is required. Such a reconstruction processor is disadvantageously slow due to the large number of computations involved. Moreover, numerous revolutions are needed to acquire the full set of rays through a given voxel. Hence, many past conebeam reconstruction processors have neglected to account for many three-dimensional effects, thus providing faster image reconstruction at the cost of degraded image quality, particularly for large cone angles.
The present invention contemplates an improved CT imaging apparatus and method which overcomes the aforementioned limitations and others.
According to one aspect of the invention, an image reconstruction method is disclosed for reconstructing cone or wedge-beam computed tomography projection data. Projection data is weighted based on at least one of its angular orientation and its location within a detector aperture. The weighted projection data is reconstructed to form a volume image representation.
According to another aspect of the invention, a computed tomography imaging apparatus for reconstructing cone or wedge beam projection data is disclosed. A weighting means is provided for weighting cone or wedge projection data based on at least one of its angular orientation and its location in a detector aperture. A reconstructing means is provided for reconstructing the weighted projection data to form a volume image representation.
One advantage of the present invention is that it improves temporal and spatial resolution of axial, spiral, as well as continuous conebeam CT images. One particular advantage of these improvements is reduced motion artifacts.
Another advantage of the present invention resides in selective application of detector aperture and angular weighting functions in reconstructing spiral conebeam data for applications such as cardiac imaging.
Another advantage of the present invention resides in efficient and flexible combination of complementary or redundant conebeam projection data from one or more half-cycles apart (i.e., combination of data that is congruent modulo 180xc2x0). Data combination is advantageously applicable in axial, spiral, and conebeam imaging. The combining is flexible to permit maximum use of the available reconstruction pipelines separately or in weighted combination.
Another advantage of the present invention resides in efficient mapping of projection data in the axial or Z-dimension onto the backprojection matrix using a non-linear recursive model particularly appropriate for conebeam geometries. The mapping process optionally incorporates planar, volume or other image sub-matrices.
Yet another advantage of the present invention resides in accommodation of data of varying CT acquisition geometries such as full-pitch conebeam, half-pitch conebeam, cardiac spiral conebeam, and continuous conebeam CT by selecting appropriate angular and aperture weighting values. The present invention is compatible with a wide range of conebeam reconstruction geometries, including n-PT geometries, wedge geometries, and the like.
Still yet another advantage of the present invention is compatibility with gated or dose modulated CT imaging.
Numerous additional advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiment.